The present invention relates generally to ionic liquids (molten salts with generic structures NR4+Xxe2x88x92, PR4+Xxe2x88x92, SR4+Xxe2x88x92, where NR4+, PR4+, SR4+ are ammonium, phosphonium and sulfonium cations, respectively) that are liquid at temperatures xe2x89xa6150xc2x0 C.; mixtures containing ionic liquids, or where the mixture contains at least one ionic liquid and one or more ionic solids; and solutions of ionic liquids in molecular (non-ionic) liquids; or combinations thereof, for the generation of electroactivity in conjugated polymers and conjugated oligomers and, more particularly, to the development of stable conjugated polymer electrochemical devices incorporating ionic liquids, such as electrochemical actuators, electrochromic devices, batteries, electrochemical capacitors, light emitting electrochemical cells, fuel cells, sensors, and photoelectrochemical solar cells.
In the development of electrochemical devices, electrolyte plays an important role. Good electrolytes have high conductivity, large electrochemical windows, excellent thermal and chemical stability, low viscosity and negligible vapor pressure. The advantages of organic electrolytes over aqueous ones for the fabrication of electrochemical devices have been appreciated. However, the operating voltage and lifetime of the resulting electrochemical devices are affected by the water content of the electrolyte due to the miscibility of water with the organic solvents employed. Additionally, the evaporation of the organic solvents limits the long-term operation of the devices. Thus, lack of volatility and lack of miscibility with water are critical properties of electrolytes for the fabrication of durable and stable electrochemical devices, especially those devices with open configuration structures such as actuators.
To ensure the high performance of conjugated polymer based electrochemical devices, the electrolyte should also contain ions capable of enhancing the electroactivity of conjugated polymers.
Ionic liquids are not new; for example, ethylammonium nitrate ([EtNH3][NO3]), which has a melting point of 120C, was first described in 1914 (see, e.g., P. Walden, Bull. Acad. Imper. Sci. (St. Petersburg), p. 1800 (1914); and S. Sugden and H. Wilkins, J. Chem. Soc., p. 1291 (1929)). Typically, ionic liquids consist of nitrogen-containing organic cations and inorganic anions. Since they are nonvolatile and nonflammable, have high thermal stability, and are relatively inexpensive to manufacture, ionic liquids have found use in chemical syntheses, particularly catalysis, and in separation technology.
Ionic liquids are inherently ionically conductive; that is, they have high conductivity and large electrochemical windows; that is, the electrochemical potential range over which the electrolyte is not reduced or oxidized at an electrode. These features make ionic liquids good electrolytes. Moreover, their low volatility and nonflammable properties are important for the fabrication of stable electrochemical devices. When compared with other electrolytes, ionic liquids have the advantage that they can be obtained in a very dry state which makes them especially suitable for applications in electrochemical systems from which moisture must be excluded over long periods of operation. Moreover, the thermal stability and low volatility of ionic liquids permit the operation of electrochemical devices under high temperature and high vacuum. There are many ways of combining different cations and anions to make ionic liquids. Therefore, high electroactivity for conjugated polymers which results in high performance, conjugated polymer electrochemical devices, can be generated by optimizing the composition of ionic liquids used therewith.
Ionic liquids have received considerable attention as electrolytes in various electrochemical devices (see, e.g., A. B. McEwen et al., Electrochemical Capacitors II, F. M. Delnick et al., Editors, PV 96-25, p. 313; V. R. Koch et al., J. Electrochem. Soc. 142, L116 (1995); V. R. Koch et al., J. Electrochem. Soc. 143, 788 (1996); J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc. Commun., p. 965 (1992); R. T. Carlin et al., J. Electrochem. Soc. 141, L73 (1994); P. Bonhxc3x4te et al., Inorg. Chem. 35, 1168 (1996); and N. Papageorgiou et al., J. Electrochem. Soc., 143, 3099 (1996)). However, the use of ionic liquids as electrolytes for the fabrication and development of conjugated polymer electrochemical devices has not been previously addressed. Nevertheless, the study of ionic liquids as electrolytes in conjugated polymer electrochemistry is very limited. Osteryoung et al. have shown that a number of electroactive polymers (polypyrrole, polythiophene and polyaniline) can be prepared in ionic liquids and the obtained polymer films showed electroactivity in these ionic liquids (see, e.g., P. G. Pickup and R. A. Osteryoung, J. Am. Chem. Soc. 106, 2294 (1984); P. G. Pickup and R. A. Osteryoung, J. Electroanal. Soc. 195, 271 (1985); L. Janiszewska and R. A. Osteryoung, J. Electrochem. Soc. 134, 2787 (1987); L. Janiszewska and R. A. Osteryoung, J. Electrochem. Soc. 135, 116 (1988); and J. Tang et al., J Phys. Chem. 96, 3531 (1992)). The research described in these papers was performed using AlCl3-1-ethyl-3-methylimidazolium chloride systems. It has been stated (U.S. Pat. No. 5,827,602 which issued to V. R. Koch et al.) that a disadvantage of these ionic liquids, and a problem with any ionic liquid containing a strong Lewis acid such as AlCl3, is the liberation of toxic gases when they are exposed to moisture. Moreover, in xe2x80x9cElectrochemistry of polyaniline in ambient-temperature molten saltsxe2x80x9d by Jinsong Tang and Robert A. Osteryoung, Synth. Met. 44 (1991) p. 307-319, the authors state that the PANI deteriorates in the ionic liquid. Thus, the highly reactive nature of Lewis acids used to form room-temperature melts limits the kinds of organic and inorganic compounds which are stable in these media, and such salts typically decompose below 150xc2x0 C.
In xe2x80x9cFormation and electrochemistry of polyaniline in ambient temperature molten saltsxe2x80x9d, by Jinsong Tang and Robert A. Osteryoung, Synth. Met. 45 (1991) p. 1-13, the authors state that ambient-temperature molten salts consisting of a mixture of aluminum chloride with 1-ethyl-3-methyl-imidazolium chloride ([EMIM][Cl]) or N-butylpyridinium chloride ([BPY][Cl]) are viewed as promising materials for secondary battery electrolytes. However, the preparation of electroactive (PANI) in molten salt systems of different compositions is described, and the cooperation of conjugated polymers with ionic salts to form electroactive devices is not discussed.
Accordingly, it is an object of the present invention to use ionic liquids and solutions containing ionic liquids as electrolytes to enable electroactivity in conjugated polymer devices.
Another object of the invention is to provide stable conjugated polymer electrochemical devices such as electrochemical actuators, electrochromic devices, batteries, electrochemical capacitors, light emitting electrochemical cells, fuel cells, sensors, and photelectrochemical solar cells using ionic liquids in conjunction with conjugated polymers.
Yet another object of the invention is to produce electroactivity in polyaniline in ionic liquids and to provide polyaniline-based durable and stable electrochemical actuators, batteries and electrochemical capacitors using ionic liquids.
Yet another object of the invention is to produce electroactivity in polyaniline and polythiophene in ionic liquids and to generate polyaniline and polythiophene-based durable and stable electrochromic devices.
Still another object of the present invention is to combine the unique properties of conjugated polymers (for example, light weight, low cost, redox reversibility, and high charge capacity) and ionic liquids (for example, high conductivity, wide electrochemical window, negligible evaporation, and chemical and thermal stability) to fabricate secondary batteries with high performance and long lifetime.
Another object of the invention is to provide a synthesis for thin film conjugated polymers.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method for inducing a response in a long-lived electrochemical device hereof includes the steps of: contacting a conjugated polymer working electrode and a counter electrode with an ionic liquid having an anion and a cation; and applying a voltage between the working electrode and the counter electrode, whereby the response is induced in the long-lived electrochemical device.
Preferably, the ionic liquid is stable in the presence of water and the conjugated polymer is stable in the presence of the ionic liquid.
In another aspect of the invention in accordance with its objects and purposes, the long-lived electrochemical device hereof includes in combination: a conjugated polymer working electrode; a counter electrode; an ionic liquid having an anion and a cation in contact with both the working electrode and the counter electrode; and an electronic power supply for applying a voltage between the working electrode and the counter electrode, whereby a response is induced in the electrochemical device.
Preferably, the ionic liquid is stable in the presence of water and the conjugated polymer is stable in the presence of the ionic liquid.
Benefits and advantages of the present invention include the flexibility of structure and function of ionic liquids which result from combining cations and anions having different properties. Since the composition of ionic liquids has a significant influence on the electroactivity and actuation of conducting polymers, by changing the ionic liquid cation and more effectively, the anion, ion exchange and thus the behavior of conjugated-polymer devices may be varied. Moreover, ionic liquids have high chemical stability, water immiscibility, negligible vapor pressure, and wide electrochemical windows. Therefore, conjugated polymer devices using ionic liquids as electrolytes have been found to have great stability and long life. Electrochromic devices using conjugated polymer electrodes and ionic liquids have also been found to consume very low electrical power.